The fungus Coelomomyces dodgei has a complex life cycle involving an alternation of generations between an intermediate copepod host, Cyclops vernalis, and a definitive mosquito host, Anopheles quadrimaculatus. Meiospores released from sporangia after mosquito death invade copepods, nauplii being the most susceptible to infection. Within the copepod a gametophyte develops and forms either orange or amber gametangia. These gametangia release similarly pigmented gametes. The different colors represent opposite mating types, mosquito-infective zygotes being formed only by the fusion of orange and amber gametes. Limited mass production of infected copepods has been achieved by exposing populations (5 x 10 to the 3rd power nauplii) approximately 48 hour old, to meiospores at a ratio of 10 to the 3rd power meiospores/nauplius. Infection is patent in many individuals at 7 days post-infection, but the population infection peak normally occurs on day 10 or 11 P.I. when 300-700 infected adults are collected per day. Mean infection rates averaged 31%. A chemically defined salt solution consisting of NaCl (.1 gm/L), KCL (.004 gm/L), CaCl2 (.006 g./L) in distilled water has been developed to study the infection of nauplii by meiospores and larvae by zygotes. In both cases the invading stages appear to enter the host via the cuticle. Infection rates obtained with the above medium are equal to rates obtained with media used previously which were chemically undefined.